From China and Tibet, by Robson Lowe. 
Monograph, commentary on twelve letters written by French and Italian missionaries in Tibet and China between 1844 and 1865.  All the letters are addressed to the director of "l'oeuvre de Sainte-Enfance" (The work of the Holy Children) in Paris, France.
Six of the letters show evidence of being carried by the postal service over some part of their journey to France. The men who wrote these letters were of varied character, and a bit misguided by today's standards. 
Very interesting short pamphlet of 20 pages.  Published 1981.